Taxa on This Page

The Chondrostei: History, Phylogeny, and Polyploidy

The Chondrostei are cartilagenous fish,
but not sharks. They are early actinopterygians, definitively known from
the Early Triassic.
Persistent reports of much earlier (Mississippian) chondrostian-like
parasphenoids (e.g., Štamberg
& Zajíc, 2000), and similar odd pieces may eventually push this
date back considerably. Since the Chondrostei have few bones, it is not
surprising that the fossil record is unreliable. There are 25 extant
species, including 23 sturgeon species and two species of paddlefish. All
extant chondrosteans spawn in rivers with moderate flow and hard beds, but --
with rare exceptions -- spend their adult lives outside the spawning area.
Sillman et al.
(2005).

The Chondrostei have been through a number
of cycles of classification. They were originally classified with the sharks.
This is not as unreasonable as might seem. Recent chondrosteans share a number
of features with recent chondrichthyans, including the (not quite!) absence of
endochondral bone, a peculiar type of jaw suspension with a protrusible palatoquadrate,
a pronounced rostrum, an antero-ventrally oriented mouth, minimal squamation,
and a tendency to be really big predators near the top of the local food chain.
By the mid-nineteenth century, however, it became clear that this sort of scheme
wasn't going to work well for fossil forms, such as Chondrosteus which
was first described in 1844.

The taxon then became the flagship of a new classification scheme based on
degrees of ossification, in which the Chondrostei were believed to be the most
primitive of osteichthyans. Although the classification scheme did not last, the
"primitive" label stuck. By the mid-twentieth century, the Chondrostei
had become a garbage taxon full of "paleonisciform" fish of uncertain
affinities.

Largely due to the efforts of Prof. William
Bemis of the University of Massachusetts [a disparaging reference to his
aging web page has been deleted as it has finally been updated as of 11/02], Lance
Grande of the Field Museum (whose web page is even more out of date), and
their students and co-conspirators, the taxon has been stripped -- as it were --
to the bones and given a proper cladistic foundation. Bemis and Grande use the
name "Chondrostei" rarely, preferring the less ambiguous "Acipenseriformes."
However, there is still some utility in a stem taxon defined as fish more
closely related to caviar (Huso) than to lox (Salmo), and it
is in that sense that "Chondrostei" is used here. Bemis's most recent
scheme is the following:

In spite of the long history of confused classification, the evolution of the
Chondrostei makes fairly good sense. In the early Mesozoic, a number of fish
families were beginning to develop adaptations which would make them lighter,
faster and more efficient predators. Generally speaking, these adaptations
involved a lighter squamation and specialized jaws allowing the back
(posterior) of the maxilla and/or premaxilla to swing down and out so that the
whole jaw apparatus was shot forward to suck in prey. The chondrosteans branched
off fairly early on, probably towards the middle of the Triassic, and
consequently tried their hand at these developments without the elaborate
structures developed by later, neopterygian groups. That is, for whatever reason
(see below), they seem to have felt the hand of these selective pressures a bit
earlier than some other groups.

Neopterygians gradually developed lighter scales and lighter, acellular types
of bone. The chondrosteans simply dropped their heavy scales, almost completely,
and omitted ossification of cartilage altogether. Elderly individual
chondrosteans still develop centers of endochondral ossification, so we may
suppose that that ossification of cartilage was simply slowed down relative to
other developmental programs. Neopterygians developed various different, complex
mechanisms for allowing the maxillary bones to attach to the palate anteriorly
so that they could shoot the jaw bones forward as they opened the jaw.
Chondrosteans again took a radical, but conceptually simpler approach. They
simply lost the maxillary bones completely. The teeth are attached to the
dermopalatines and the entire palatoquadrate essentially drops off the braincase
and takes everything else with it, remaining in articulation only through the
hyomandibula.

Why did Chondrosteans react so quickly, in evolutionary terms, and with such
total reorganization? Undoubtedly there are a number of different answers which
apply, but one answer surely has to be the way in which they evolved.
Chondrosteans, like only a few other vertebrate groups, have clearly undergone
several rounds of polyploidy. Normal vertebrate cells have two of each gene, a
maternal set and a paternal set. They are referred to as diploid, i.e. with two
of each kind of chromosome (the cellular structures which contain each cell's
complement of DNA), or "2n". For a variety of reasons, a cell may
occasionally replicate its DNA complement but fail to divide during that cycle.
It ends up with four of each gene. Such cells are tetraploid, or
"4n". If this occurs during the first steps of development, an entire
organism may become 4n and pass the trait on to its progeny. Normally such
sports are not viable in the animal kingdom, or are sterile. However, a few
types of organisms appear to be able to survive, or even flourish with polyploid
genomes. Chondrosteans or one of those types. Indeed, recent chondrosteans are
8n or 16n. Such a dramatic genetic change undoubtedly creates the potential for
sudden, drastic reorganization of the body plan.

Thus, it is not surprising that the Chondrostei "reacted" more
quickly to the general selective pressures facing Mesozoic fish, and that they
reacted by reorganizing their existing structures rather than, as was the case
with neopterygians, by gradually evolving new and different structures.

Image:Huso huso (beluga) from Fitzinger, LJ &
J Heckel (1836), as stated in Bemis et
al. (1997). Lower image, Cheirodopsis (Mississippian of England) from Paleozoic
Fish (I'm not at all convinced of the date or identification, but a
wonderful site nonetheless!).

Note: widely stated to have been a top predator, despite
sometimes relatively small size. This observation may be based in part on its
very broad ecological range and the fact that it is seldom (never?) found in
groups. Based on body shape, it is believed to have been capable of high
accelerations and possibly ambush predation. The physiological basis for this
conclusion is discussed in detail at Saurichthys.
However, ambush predators are more likely to specialize in a particular type of
terrain and hence a limited range. Nevertheless, the physical similarity to,
e.g. the modern pike (Esox) is striking.